PROJECT SUMMARY The neural substrates underlying motor recovery after stroke are poorly understood. Despite decades of research, strategies for optimizing recovery remain lacking, possibly because of the failure to consider non- corticospinal pathways as potential recovery substrates. The corticoreticulospinal tract (CReST) is a bilaterally descending motor pathway in humans. In animals that have recovered from corticospinal tract injury, the contralesional CReST shows functional and structural upregulation. In humans that have recovered from corticospinal stroke, contralesional CReST neurophysiology shows increased excitability. These findings suggest that the contralesional CReST has the capacity to reorganize after stroke, but it is not known if these changes directly relate to motor improvement. The overall objective of this application is to identify the role of the contralesional CReST in motor recovery. Our central hypothesis is that functional and structural changes in the contralesional CReST will causally relate to motor recovery in the upper extremity. The rationale underlying the proposed research is that, once a recovery role is identified, the CReST could be manipulated to accelerate recovery. In the first six months following ischemic stroke, we will longitudinally measure strength, motor control, and motor synergies to characterize motor recovery. We will use transcranial magnetic stimulation (Aim 1) and structural MRI (Aim 2) to precisely detail contralesional CReST neurophysiology and microstructure. Pathway-behavior relationships will be assessed within a causal inference framework. We expect to show that contralesional CReST reorganization, manifesting as increased excitability and tissue complexity, will relate to increasing strength, motor control, and synergy expression after stroke. The proposed work is significant, because it is expected to provide strong scientific justification for targeting the contralesional CReST to potentiate recovery. The proposed work is innovative, because it combines advanced complementary approaches to focus on a surprisingly understudied pathway in motor recovery. Our study is likely to have a positive impact on the field of neurorehabilitation, because it will vertically advance our understanding of motor recovery mechanisms, leading to the development of rationally designed therapies to improve stroke outcomes.